The proposed work is a characterization of wild type and mutant leucyl-tRNA synthetases from the model mammalian system, Chinese hamster ovary (CHO) cells in culture. This study also involves functional roles for the large particulate aminoacyl-tRNA synthetase complexes which exist in mammalian cells. We are currently characterizing the defective leucyl-tRNA synthetase found in the temperature sensitive CHO mutant, tsHl. This enzyme has altered kinetic properties and an altered leucyl-tRNA synthetase complex with the other aminoacyl tRNA synthetase complexes being unchanged. We are also characterizing aminoacyl-tRNA synthetase complex alterations as a function of cell cycle and cellular growth conditions.